Microelectromechanical systems (“MEMS,” hereinafter “MEMS devices”) are used in a wide variety of applications. For example, MEMS devices currently are implemented as microphones to convert audible signals to electrical signals, as gyroscopes to detect pitch angles of airplanes, and as accelerometers to selectively deploy air bags in automobiles. In simplified terms, such MEMS devices typically have a movable structure suspended above a substrate, and associated electronics that both senses movement of the suspended structure and delivers the sensed movement data to one or more external devices (e.g., an external computer). The external device processes the sensed data to calculate the property being measured (e.g., pitch angle or acceleration).
As their name suggests, MEMS devices are very small. Consistent with this goal, the movable structure in a MEMS device has a very small mass. For example, the mass of the movable structure may be on the order of one microgram. Such small structure, when implemented in an accelerometer, produces small inertial forces. When subjected to an acceleration, the resulting displacement may be insufficient to detect unless the structure is held with very compliant springs. Very compliant springs, however, are fragile and may not provide enough return force if the structure contacts other internal components (i.e., the structure is more susceptible to stiction problems).
On solution to this problem is to increase the mass of the structure relative to the substrate. Of course, increasing the size of the structure is inconsistent with the goals of MEMS devices; namely, reducing the overall size of the final device.